U.S. patent number 4,600,568 [Application Number 06/714,742] was granted by the patent office on 1986-07-15 for flue gas desulfurization process.
This patent grant is currently assigned to Conoco Inc.. Invention is credited to Robert M. Statnick, Heeyoung Yoon.
United States Patent |
4,600,568 |
Yoon , et al. |
* July 15, 1986 |
Flue gas desulfurization process
Abstract
Flue gas is desulfurized by mixing into it at a temperature
between about 120.degree. and about 230.degree. C. a finely divided
sorbent such as calcium hydroxide and, immediately downstream of
the mixing point, spraying the gaseous suspension with an aqueous
solution of solubilizing agent such as a deliquescent compound or a
strongly ionizing inorganic salt. In one embodiment, make-up
sorbent is derived from injection of limestone or dolomite into the
combustion zone, and in another embodiment a portion of the solids
separated from the flue gas is reactivated for reuse as
sorbent.
Inventors: |
Yoon; Heeyoung (McMurray,
PA), Statnick; Robert M. (Pittsburgh, PA) |
Assignee: |
Conoco Inc. (Wilmington,
DE)
|
[*] Notice: |
The portion of the term of this patent
subsequent to July 15, 2003 has been disclaimed. |
Family
ID: |
24871280 |
Appl.
No.: |
06/714,742 |
Filed: |
March 22, 1985 |
Current U.S.
Class: |
423/244.08;
423/242.3; 423/242.4; 423/244.07 |
Current CPC
Class: |
B01D
53/508 (20130101); B01D 53/501 (20130101) |
Current International
Class: |
B01D
53/50 (20060101); C01B 017/00 (); B01J
008/00 () |
Field of
Search: |
;423/242A,242R,243,244A,244R ;110/343,345 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3632306 |
January 1972 |
Villiers-Fisher et al. |
3798309 |
March 1974 |
Knowles et al. |
4080428 |
March 1978 |
Holter et al. |
4454102 |
June 1984 |
Lindau et al. |
4533532 |
August 1985 |
Gebhard et al. |
|
Primary Examiner: Heller; Gregory A.
Attorney, Agent or Firm: Mikesell; William A. Schupbach;
Cortlan R.
Claims
What is claimed is:
1. The method for reducing sulfur dioxide content of a flue gas
resulting from combustion of a sulfur-containing fuel, which method
comprises:
(a) mixing into said flue gas, at a point where its temperature is
between about 120.degree. and about 230.degree. C., a finely
divided dry sorbent comprising alkaline earth metal oxide or
hydroxide in amount sufficient to provide a metal salt: sulfur
ratio of at least about 0.5, said alkaline earth metal being
selected from calcium and magnesium;
(b) spraying into the resulting suspension of sorbent in flue gas
an aqueous solution of solubilizing agent, such agent being
selected from sodium hydroxide, sodium carbonate, calcium chloride,
adipic acid and glycerol;
(c) providing a contact time between said sorbent in flue gas and
droplets resulting from said spraying of at least about 1
second;
(d) subsequently separating from said flue gas solids comprising
sorbent and solids resulting from combustion of said fuel;
(e) discharging from said separating a flue gas of substantially
diminished sulfur dioxide content; and
(f) regulating the rate of said spraying relative to the rate of
said flue gas such that the temperature of said flue gas at the
point of said separating is between about 10.degree. C. and about
35.degree. C. above its saturation temperature.
2. The method of claim 1 wherein additional sorbent selected from
calcium and magnesium carbonate is introduced in dry finely divided
form into a zone adjacent said combustion.
3. The method of claim 2 wherein a portion of the solids derived
from the separating of step (d) is admixed with water in at least
about stoichiometric amount to hydrate its alkaline earth oxide
content, and the resulting hydrate in dry state is returned as
sorbent to step (a).
4. The method of claim 1 wherein said solubilizing agent comprises
sodium hydroxide in the amount of about 5 to about 10 weight
percent of said sorbent.
Description
FIELD OF THE INVENTION
This invention is directed to an improved process for reducing the
sulfur content of flue gas derived from combustion of a
sulfur-containing fuel, in which process first a dry sorbent and
subsequently an aqueous solution containing a solubilizing agent is
sprayed into such flue gas. Ash and sorbent are subsequently
separated from the flue gas. In one embodiment, a portion of the
separated solids is regenerated and returned as sorbent, and in
another embodiment the sorbent is initially injected into the
boiler for activation by calcination.
BACKGROUND OF THE INVENTION
Efforts to reduce sulfur emissions in the gaseous products from
combustion of a sulfur-containing fuel have been made in varying
directions. Some processes attempt to reduce or eliminate the
sulfur in the fuel prior to its combustion. Other processes propose
the addition of compounds to the combustion zone which will in some
manner change the nature of the sulfur compounds such that they may
be more readily removed from the combustion products. And yet other
processes remove sulfur compounds from the gaseous combustion
products by chemical reaction.
U.S. Pat. No. 4,185,080, issued Jan. 22, 1980 to Rechmeier
discloses a combustion gas desulfurization process wherein a
powdered sorbent such as limestone or dolomite is added to a
combustion zone, and a portion of the solids collected from the
flue gas is reactivated and can be returned for injection into or
downstream of the combustion zone.
The approach of chemical treatment to effect flue gas
desulfurization can be further subdivided into wet scrubbing
wherein a solution or suspension of reagent both enters and leaves
the flue gas contacting zone in liquid state, spray drying wherein
a solution or suspension of reagent enters the flue gas contacting
zone in liquid state but is dried to produce a powdered solid
leaving the contacting zone, and dry treatment wherein the treating
reagent is a solid state powder both entering and leaving the
contacting zone.
Illustrative of the wet scrubber approach is U.S. Pat. No.
3,928,537, issued Dec. 23, 1975 to Saitoh et al, which discloses
contacting the exhaust gas with an aqueous solution of an organic
acid to form a soluble sulfite or sulfate. The sulfite or sulfate
is removed, and the organic acid regenerated, by a second step
comprising reaction with a calcium compound such as an inorganic or
an organic acid salt, for example calcium hydroxide or calcium
formate.
The spray dryer approach is illustrated for example by U.S. Pat.
No. 4,279,873, issued July 21, 1981 to Felsvang et al, which
discloses spraying a suspension of fresh slaked lime and recycled
fly ash plus spent calcium compound into the hot flue gas in such a
manner as to evaporate the slurry droplets to dryness; the
resulting powdered solids are removed from the flue gas by a
downstream electrostatic precipitator or bag filter.
U.S. Pat. No. 4,178,349, issued Dec. 11, 1979 to Wienert
illustrates the dry treatment; it discloses mixing a dry, powdered
lime-bearing material in a reactor, and subsequently separating the
solids from the treated flue gas. Another patent, U.S. Pat. No.
4,442,079, issued Apr. 10, 1984, to Donelly et al, outlines a flue
gas desulfurization process which is primarily adapted to the spray
dryer procedure just discussed, but is also stated to be applicable
to injection of dry sorbent at a point of relatively low flue gas
temperature, with water being sprayed into the gas either upstream
or downstream of the sorbent injection point.
Current thinking seems to be that no one of the above-discussed
strategies is the unique answer to the sulfur emission problem,
either for new installations or for retrofit on an existing
installation. Rather, numerous site-specific factors such as
proximity to reagent source, space availability, and extent of
sulfur removal required, must enter into the selection at each
plant.
SUMMARY OF THE INVENTION
It is an objective of the present invention to provide a novel and
advantageous method of removing sulfur compounds, especially sulfur
dioxide, from combustion exhaust gas.
According to the present invention, flue gas is desulfurized by
first contacting it at a relatively low temperature with a finely
divided dry sorbent, and then humidifying the resulting gaseous
suspension of solids by spraying therein a solubilizing solution.
Solids subsequently removed from the gas can be reactivated for
recycle. In another embodiment, make-up sorbent is initially
activated by injecting it into the combustion zone.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing, the sole FIGURE represents a flow diagram,
partially schematic, of a flue gas desulfurization process
according to our invention.
DETAILED DESCRIPTION OF THE INVENTION
Flue gas containing sulfur dioxide from combustion of a
sulfur-containing fuel is typically passed from the combustion
chamber through heat recovery exchangers and thence by way of duct
work to a stack for disposal. Where the fuel is a solid, such as
coal, or a heavy liquid, the combustion product gases will also
contain ash solids such as fly ash, in which instance the flue gas
will first be treated for solids removal by such as a bag filter or
electrostatic precipitator (ESP) prior to being passed to the
stack.
The invention will now be described by referring to the drawing. A
combustor such as boiler 10 is provided with a burner 12 which is
adapted to introduce a pulverized solid fuel such as coal from
conduit 14 and a combustion-supporting gas such as air from cnduit
16. Hot combustion products leave the upper region of boiler 10,
travelling past and through various heat exchange surfaces which
typically extract heat by radiant and convective transfer, until
they are discharged from the boiler assembly at duct 18. The final
stage of heat recovery from the flue gas is often an air preheater,
shown here as 20, which serves to preheat combustion air to conduit
16. Past practice has been that flue gas leaving air preheater 20
by way of duct 22 is passed directly to solids removal to separate
out fly ash, such as by a bag filter or electrostatic precipitator
24, and thence discharged by a stack 26.
According to our invention, the hot flue gas in duct 22 is treated
for capture of its sulfur dioxide and sulfur trioxide content by
injection of a finely divided dry sorbent powder such as calcium
hydroxide, magnesium hydroxide, or mixtures of these two, by way of
a conduit 28. The dry powder can be carried through conduit 28 by
use of a transport gas such as air or superheated steam from
conduit 30. A short distance downstream from conduit 28, duct 22
enters humidifier 32, wherein solubilizing solution from conduit 34
is sprayed into the flue gas. Humidified flue gas passes by way of
duct 36 to solids collector 24 and thence to stack 26. A portion of
the flue gas in duct 22 can by-pass both the sorbent powder
addition of conduit 28 and humidifier 32 by way of by-pass duct 38
and its associated damper 40, as will be more fully discussed.
Solids removed in collector 24 will normally comprise a mixture of
fly ash, spent sorbent, and a proportion of unreacted sorbent.
According to one embodiment of our invention, one portion of these
collected solids is discarded to waste by way of conduit 42, and
another portion in conduit 44 is treated to regenerate and recycle
its unreacted sorbent content. This portion can be subjected to
grinding as by mill 46, although such is usually not required, and
can then be passed by way of conduit 48 to slaker 50, if necessary,
or can by-pass slaker 50 directly to conduit 28 by a by-pass
conduit not shown. The unreacted sorbent portion of the solids in
conduit 48 is usually in the form of calcium and/or magnesium
hydroxide. However, in one embodiment this portion can contain the
oxide, and this material is reacted in slaker 50 with sufficient
excess over the stoichiometric amount of water or preferably steam
from conduit 52 to produce a dry slaked hydroxide product. As
described earlier, this portion of solids containing dry slaked
sorbent becomes the material injected by way of conduit 28 into
duct 22.
Sorbent values, both spent and unspent, are continuously discarded
from the system by way of conduit 42. Make-up sorbent can be added
to the system according to two different embodiments of our
invention. In one embodiment, make-up is added in the form of the
oxide, e.g. quicklime, to slaker 50 by way of conduit 54. In the
other embodiment, the sorbent is added in its less expensive but
less reactive carbonate form, as limestone or dolomite, at conduit
56. This material can either be added to boiler 10 by way of
conduit 58 into burner 12, or by way of conduit 60 through separate
injection ports into an upper region of the combustion zone,
depending upon the prevailing temperature profile and residence
time. In either event, the limestone or dolomite is calcined by the
elevated temperature. The resulting suspension of calcium and/or
magnesium oxide is thus afforded additional reaction time for flue
gas desulfurization in duct 18 and air preheater 20.
The various operating parameters which affect the efficacy of our
invention will next be examined.
The process of the present invention is especially adapted to
remove sulfur dioxide from a flue gas wherein availability of
relatively short residence time in the conduct work upstream of the
ESP (as in a retrofit installation) and/or a relatively high
concentration of sulfur dioxide combine to present difficult
removal. The humidification of the flue gas and its entrained
sorbent powder by spraying water containing solubilizing agent is
an essential aspect of our invention. As solubilizing agent we
presently prefer sodium hydroxide, although other useful materials
include sodium carbonate, calcium chloride, adipic acid, and
glycerol. Although we do not wish to be so bound, we theorize that
compounds which form highly ionized solutions or which are
deliquescent serve to form a thin liquid film of greatly increased
reactivity toward sulfur dioxide on the surface of each of the
particles of gas-suspended solid sorbent.
It is necessary for several reasons to control the humidity, or
approach to dew point, of the flue gas containing sorbent and
sprayed solution. It is preferable to approach the dew point at the
inlet to the solids collector as close as possible without causing
operating problems, since we postulate that a more humid flue gas
permits existence of a liquid film on the sorbent, and the reaction
of gaseous sulfur dioxide with liquid solution is much more rapid
than with a solid particle. On the other hand, if the dew point is
approached too closely, the spray droplets tend to foul and plug
the surfaces of bag filters or ESP collector plates, and to cause
excessive corrosion of duct work. We prefer to approach at least to
within about 35.degree. C. of, but no nearer than about 10.degree.
C. of, the saturation temperature. According to one embodiment of
our invention, the degree of approach to saturation is controlled
by proportioning the amount of flue gas admitted through duct 38 by
damper 40.
The degree of desulfurization achieved is also controlled by the
Ca/S ratio, i.e. the amount of calcium (and/or magnesium) compound
in the sorbent as compared to the amount of sulfur dioxide in the
flue gas being treated. This ratio is normally expressed in moles,
and we prefer that it be in the range from about 0.5:1 to about
3:1. It is controlled by the flow rates in conduits 28, 54 and/or
56. Solubility of the solubilizing salt, e.g. calcium chloride,
sodium hydroxide, etc. in water determines the maximum
concentration of spray solution. It is advantageous that the duct
work provides a residence time for the humidified solids of at
least about 1 second, and preferably more.
In the usual application of this invention, the sulfur-containing
fuel being burned will also contain a significant proportion of
ash-forming constituents, and thus the flue gas will contain fly
ash. Addition of solubilizing (humidifying) solution and dry
sorbent according to the present invention causes an increase in
the solids loading of the flue gas, and thus on the duty to be
served by the downstream solids collector bag house or ESP.
However, it has been determined that the solids retained on the
surfaces of such solids collector remain active for SO.sub.2
absorption for a longer period when humidifed with a solubilizing
agent according to the present invention, particularly when the
desired close dew point approach is maintained. Additionally, the
collected solids contain some non-reacted calcium values, in
addition to the fly ash and sulfite and sulfate compounds, which
values can be reclaimed, as noted, by recycle.
The temperature of the flue gas at the point of injection of the
dry sorbent via conduit 28 should be between about 120.degree. and
about 230.degree., and preferably between about 140.degree. and
about 175.degree. C.
The invention will now be illustrated by the following
examples.
EXAMPLE I
A flue gas from an electrical utility station burning 2.4 weight
percent sulfur coal is analyzed at a point in conduit 22 just
downstream of the air preheater. The flue gas analysis is as
follows: temperature 150.degree. C., water content 5 volume
percent, sulfur dioxide 1750 ppm (by volume, dry basis), oxygen
4.5%, and solids content 2.5 grains/scf (dry). Into this flue gas
is injected by way of conduit 28 an air suspension of finely
divided slaked lime in amount sufficient to provide a resultant
Ca:S molar ratio of about 2. Next, a solution of sodium hydroxide
in water is sprayed into the gas stream by way of conduit 34 in
amount sufficient to result in a gas temperature about 10.degree.
C. above the dew point at the ESP inlet and a weight ratio of NaOH
to Ca(OH).sub.2 of about 0.1. The flue gas velocity in the existing
duct is such that the sorbent particles have a residence time of
about 2 seconds prior to passage into the ESP. Average residence
time of collected solids in the ESP is about 22 minutes. The
SO.sub.2 content of the gas exit the ESP is reduced to 440 ppmv,
dry basis.
EXAMPLE II
The test of Example I is repeated, except that about half the
active calcium entering humidifier 32 is recycle by way of conduit
44, and the balance is derived from limestone admitted by way of
conduit 60. The SO.sub.2 content of the gas exit the ESP is again
about 440 ppmv, dry basis.
EXAMPLE III
As a basis for comparison, Example I is repeated except that no
solubilizing agent is added in the water through conduit 34. The
sulfur dioxide analysis exit the ESP increases to 850 ppmv,
dry.
EXAMPLES IV-VI
Solutions of calcium chloride, sodium carbonate, and glycerol are
sequentially substituted for the sodium hydroxide solution in
Example I, with similar sulfur dioxide removal results.
EXAMPLE VII
Example I is repeated, except that the quantity of sodium hydroxide
solution is decreased to the point that the dew point approach is
only about 25.degree. C. Analysis of SO.sub.2 exit the ESP
increases to about 960 ppmv, dry.
EXAMPLE VIII
Example I is repeated, except that the slaked lime rate in conduit
28 is decreased to provide a Ca:S molar ratio of about 1. The
SO.sub.2 analysis exit the ESP increases to about 950 ppmv,
dry.
Reasonable variation and modification are possible within the scope
of the foregoing disclosure and the appended claims to our
invention, the essence of which is that we have provided an
improved method for reducing the sulfur dioxide content of flue gas
by reaction with sorbent of increased activity, and for preparing
such sorbent.
* * * * *